1. Field of the Invention
This invention relates to dispersion compensating fibers and to dispersion compensating fiber modules, and specifically relates to dispersion compensating fibers and to dispersion compensating fiber modules which are used to compensate dispersion in a non-zero dispersion shifted optical fiber having a chromatic dispersion of approximately a few ps/nm/km in the 1.55 μm band.
2. Background Art
As erbium doped optical amplifiers have become available, communication systems, specifically, using the 1.53-1.63 μm wavelength band, such as an ultra-long-haul non-regenerative repeater system, which utilize optical amplifiers, have been commercialized. On the other hand, as an increased transmission capacity is required, the wavelength division multiplexing transmission technology has been rapidly developed, and some transmission paths installed for this purpose have been commercialized. In the technical trend in the near future, expanding of the operating wavelength band and increasing of wavelength multiplicity will rapidly progress.
Assuming that signals should be rapidly transmitted through transmission paths, these transmission paths should preferably be optical fibers which have less chromatic dispersion in the transmission band, and which do not have zero dispersion wavelength in the operating wavelength in order to suppress nonlinearity. In addition, it is important specifically for the wavelength division multiplexing transmission systems that the gain difference between the wavelengths due to the erbium-doped optical fiber amplifier, be minimized in the operating wavelength band, and be somewhat small dispersion, and that the change ratio of the dispersion in accordance with the change in wavelength (hereinafter referred to as dispersion slope) in the entire transmission paths be kept low in order to suppress dispersion differences between the wavelengths due to the dispersion slope, and to suppress dispersion differences among the wavelengths in the operating wavelength band as much as possible.
Furthermore, because the wavelength multiplicity is greatly increased and the power of the light being transmitted through optical fibers is also greatly increased in recent long-haul transmission systems, it is essential to use a technique suppressing nonlinearity which may degrade transmission properties.
The magnitude of nonlinearity is represented by n2/Aeff, where n2 is a non-linear refractive index of the optical fiber, and Aeff is an effective area of the optical fiber. In order to suppress the nonlinearity, n2 should be reduced or Aeff should be increased; however, it is difficult to greatly reduce n2 in the case of silica based optical fiber because n2 is inherent in the material forming the optical fiber. Accordingly, the current development of nonlinearity suppressing optical fibers is focused on increasing the Aeff of the optical fiber.
Various kinds of non-zero dispersion shifted optical fibers (hereinafter abbreviated as NZ-DSF) having a chromatic dispersion of approximately a few ps/nm/km in the operating wavelength band, in which the zero-dispersion wavelength is slightly shifted from the operating wavelength band, have been installed all over the world, as well as standard single-mode optical fiber with zero-dispersion wavelength in the 1.3 μm band (hereinafter abbreviated as S-SMF) networks, and it is predicted that more such NZ-DSFs will be installed in the future. Because the chromatic dispersion of such optical fibers are suppressed to approximately +4 ps/nm/km in the 1.55 μm band, these optical fibers may be installed without compensating chromatic dispersion for longer distances than in the case of the S-SMF. When these optical fibers are used for signal transmission at a transmission rate of 10 Gb/s, the upper limit of the transmission distance due to residual dispersion is approximately 200-300 km.
Therefore, dispersion compensating fibers for compensating chromatic dispersion of the NZ-DSFs are being developed as well as dispersion compensating fibers for the S-SMF. Because these dispersion compensating fibers have a large negative dispersion and a large negative dispersion slope in the operating wavelength band due to the controlled refractive index profile, it is possible to compensate positive dispersion, generated in the S-SMF and the NZ-DSFs, over a broad wavelength range by connecting the dispersion compensating fibers having appropriate length with the transmission optical fibers, whereby a high speed transmission can be realized.
Because the NZ-DSF has less chromatic dispersion in the operating wavelength band than in the case of S-SMF, the ratio of dispersion slope of the dispersion compensating fiber for the NZ-DSF relative to chromatic dispersion to be compensated, i.e., relative dispersion slope (hereinafter abbreviated as RDS), is generally high, which makes it difficult to manufacture the dispersion compensating fiber for the NZ-DSF.
In the case of S-SMF whose dispersion properties in the 1.55 μm band are such that the chromatic dispersion is about +17 ps/nm/km, and the dispersion slope is about +0.058 ps/nm2/km, the RDS required in the dispersion compensating fiber for the S-SMF is approximately 0.0034 nm−1. On the other hand, in the case of NZ-DSF, whose dispersion properties are such that the chromatic dispersion is about +4.5 ps/nm/km, and the dispersion slope is about +0.045 to +0.090 ps/nm2/km, the RDS required in the dispersion compensating fiber for the NZ-DSF is approximately as large as 0.01 nm−1 to 0.02 nm−1; therefore, the absolute value of the negative dispersion slope in the dispersion compensating fiber must be set to be large. Specifically in the case of NZ-DSF having a large effective area among NZ-DSFs, whose dispersion properties are such that the chromatic dispersion is about +4.5 ps/nm/km, and the dispersion slope is about +0.090 ps/nm2/km, the RDS required in the dispersion compensating fiber for the NZ-DSF is approximately 0.02 nm−1, which is as large as six times the RDS required in the dispersion compensating fiber for the S-SMF. Accordingly, the absolute value of the negative dispersion slope in the dispersion compensating fiber must be set to be large.
In the past, some documents describing examples of such dispersion compensating fibers have been published. For example, U.S. Pat. No. 5,838,867 discloses an invention of a dispersion compensating fiber in which the RDS is set in a range from 0.010 to 0.013 nm−1 for the chromatic dispersion ranging from 0 to −40 ps/nm/km. Furthermore, U.S. Pat. No. 6,263,138 discloses an invention of a dispersion compensating fiber in which the RDS is set in a range from 0.0067 to 0.0069 nm−1 for the chromatic dispersion ranging below −40 ps/nm/km.
Although, these documents recognize that the preferable range for the RDS is a range greater than 0.007 nm−1, none of them discloses a manufacturing method for a dispersion compensating fiber whose RDS is set more than 0.007 nm−1, where the chromatic dispersion ranges from −50 to −130 ps/nm/km, and whose RDS is set in a range from 0.016 to 0.024 nm−1, where the chromatic dispersion ranges from −20 to −140 ps/nm/km.
Therefore, it could hardly be possible to completely compensate the dispersion slope in the optical transmission path consisting of the NZ-DSFs using the above disclosed dispersion compensating fibers, and a large residual dispersion remains at the ends of the optical transmission path. As a result, further dispersion compensation is required in order to realize a high speed transmission; however, the transmission properties may be degraded due to a large transmission loss.